Mark A. McEver

Adaptive feedback control of optical jitter using Q-parameterization

Vibration-induced jitter degrades the pointing and imaging performance of precision optical systems. Practical active jitter reduction is achieved by maintaining beam alignment with mirror-positioning control systems. In the presence of time-varying or uncertain disturbances, jitter control systems using fixed-gain feedback control loops cannot operate without significant limitations on their performance. A feedback control technique called Q-parameterization can adapt to time-varying disturbances by adjusting its parameters in real time to maintain optimal performance. Adaptive feedback jitter control using Q-parameterization is experimentally verified on an optical testbed, increasing jitter reduction compared to an H2-optimal fixed-gain controller.

Active control and closed-loop identification of flutter instability in typical section airfoil

This research used the technique of Q parameterization to identify an unstable model of an airfoil above its flutter boundary. A nominal, stabilizing controller was designed and implemented on a typical section airfoil with an articulated trailing-edge flap. Although stabilized by the nominal controller, the open-loop, unstable plant was identified from closed-loop signals at increasing flow speeds. The plant model identified at the highest flow speed was then used to design a new controller using the Evans root-locus technique. In a series of wind tunnel tests, the nominal controller increased the flutter boundary by 30% above the open-loop flutter speed, and the redesigned controller increased the boundary by 52%.

Active Flutter Control with V-Stack Piezoelectric Flap Actuator

Aeroelastic control of wings using distributed, trailing-edge control surfaces is of interest for maneuvers, gust alleviation, and flutter suppression. The use of high-energy-density, piezoelectric materials as motors provides an appealing solution to the problem of flutter suppression. A new piezoelectric actuator, the V-stack piezoelectric actuator, was designed and bench tested at Duke University. This actuator meets the requirements for trailingedge flap actuation in both stroke and force. It is compact, simple, and sturdy and leverages stroke geometrically with minimum force penalties while displaying linearity over a wide range of stroke. Integration of the actuator inside a structure requires minimal modifications. The shape of the actuator makes it extremely suitable for trailing-edge flap actuation, eliminating the need for a push rod. A typical section prototype was constructed and tested experimentally in the wind tunnel at Duke University. This experiment was designed for preliminary evaluation of the actuation concept. During bench tests the desired flap deflection of ±5 deg was obtained. Windtunnel experiments showed that air flow has little influence on flap deflection, suggesting good actuation authority. Actuator-flap frequency bandwidth achievable for this experiment, in the context of ±5 deg flap deflection, was sufficient and facilitated control design. Positive position feedback (PPF) control was used to add damping to the unstable flutter mode. Operating in closed loop, the flutter was suppressed at the speed at which the flutter occurred open loop, and the flutter speed was increased by more than 30%.

Active jitter suppression of optical structures

This paper presents the results of experiments involving jitter suppression of optical components. Acoustic disturbances and structurally transmitted vibration contribute to the jitter of optical systems such as lasers. Active and passive methods must be used to suppress jitter from entering the optical train. An experimental test bed is constructed to study the effects of acoustic disturbances on an optical system. A laser source is directed onto a light-detecting target by way of a turning mirror and fast-steering mirror (FSM). The FSM, actuated by three piezoelectric stacks, provides tilt in both the elevation and azimuth axes. Both mirrors are exposed to an acoustic disturbance. The objective is to use knowledge of the acoustic-structural interaction to design a controller that precisely points the laser. To achieve this, several control methodologies are studied. A servo control loop around the FSM is designed using an H2 approach. By feeding back the laser beam position to the FSM, the jitter is reduced by a factor of 2.5. Feedforward methods are also explored using microphones and accelerometers as disturbance sensors. Acoustic noise control is studied as a means of reducing the sound pressure level in the proximity of the optics. Sound pressure sensed by a microphone was fed to a loudspeaker and the loop was closed with an H2 optimal controller.

Experiments in adaptive optical jitter control

Optical jitter, the centroid-shifting of a light image, concerns engineers and scientists working with lasers and electro-optical systems. Even micron-level relative motion between individual optical components such as mirrors and lenses causes optical jitter, resulting in pointing inaccuracy, blurred high-resolution images, and poor nanotechnology quality. Typical jitter control technology uses fast-steering mirrors to correct for structural and acoustic disturbances in the beam train. Unknown or time-varying disturbance characteristics necessitate a controller that can adapt its parameters in realtime. The application of one such adaptive feedback controller algorithm has been proposed by the authors. The algorithm uses a technique known as Q-parameterization to structure the controller as a function of plant coprime factors and a free parameter, Q. An inherent property of this structure is the formation of a disturbance estimate based on subtraction of the controller influence from the feedback signal. The free parameter, Q, filters this estimate to form a portion of the control signal. If the controller influence on the feedback signal is estimated from accurately modeled plant dynamics, the disturbance estimate contains no feedback information allowing Q to be designed in an open-loop fashion. A gradient descent Least Mean Squares (LMS) algorithm updates the coefficients of the filter Q in realtime to minimize the frequency-weighted RMS jitter. Experiments on an optical jitter control testbed with Q set to a 200-tap digital finite impulse response (FIR) filter resulted in jitter reductions of 35% - 50%, without requiring prior knowledge of the disturbance spectrum.

ADAPTIVE CONTROL FOR INTERIOR NOISE CONTROL IN ROCKET FAIRINGS

This work describes the development and application of interior noise control techniques based on two adaptive feedback control methodologies. First, an adaptive feedback control technique based on Q-parameterization is used to augment a fixed-gain controller with an adaptive FIR filter for adaptive disturbance estimation. By adjusting the FIR filter coefficients in realtime, the controller is able to adapt to time-varying sound pressure spectrums. Second, a recursive generalized predictive control algorithm, combining the process of system identification and the process of the controller design, is presented for noise control. Identifying the model and designing the controller in realtime enables the controller to fully adapt to time-varying plant dynamics and timevarying disturbances. Experimental results, obtained from a cylindrical enclosure modeled after a launch vehicle, are used to demonstrate the effectiveness of the adaptive Q-parameterized controller and recursive generalized predictive controller. Both control strategies, while very different in implementation, essentially led to the same result, due to the limitations imposed by the physical system. These limitations include the global dynamics of the acoustic space as well as speaker and microphone positions. Details of the design procedures and experimental applications are discussed.

Adaptive feedback control using coprime factorization

An algorithm is presented which uses adaptive Q-parameterized compensators for control of stable or unstable systems. Internal stability is maintained by forming the compensator out of plant-stabilizing coprime factors, and an on-line gradient descent method adapts the free parameter to minimize the mean squared error between the desired and actual output. The adaptation algorithm is derived for a compensator in the form of a finite impulse response (FIR) filter and a lattice infinite impulse response (IIR) filter. Simulations predict good performance for both tonal and broadband disturbances, and a duct noise control experiment results in a 37 dB tonal reduction.

Aeroelastic control using V-stack piezoelectric actuator and Q-parameterized system identification

Aeroelastic control of flutter by means of trailing edge surfaces can be a very effective method, providing that the actuation system is capable of generating suffcient force and displacement over the bandwidth of interest. This effort describes the mechanical design aspects of a flap actuation system using V-stack piezoelectric actuator and Q-parameterization technique for identifying the plant at supercritical speeds. A flap actuation mechanism that takes advantage of the shape of the actuator (V) was designed. In order to validate the actuation concept the actuator was integrated into a NACA 0015 typical section that was tested in the wind tunnel at Duke University. An initial nominal controller was designed to stabilize the typical section for a limited range of speeds above the open-loop flutter boundary. The technique of Q-parameterization was then used to parameterize the unstable system as a function of stable systems, each derived from the nominal controller. Operating in closed loop, flutter was suppressed at the speed it occurred in open loop, and the flutter boundary was extended by more than 50%.

Robust, Adaptive Q-Parameterization for Feedback Control of Vibro-Acoustic Systems

An adaptive, single-mode feedback controller for vibroacoustic applications is developed based on the IMC form Q-parameterization. By structuring the Q-parameter as a third-order filter with fixed dynamics and an adaptable gain, the resulting controller is robust to changes in plant natural frequency variation. Analysis shows the advantages of using a highly-damped plant model in terms of increased stability and performance robustness, and was proven by closed-loop simulations on a single-mode plant with a time-varying natural frequency. Even with a natural frequency variation of ±50%, the adaptive-Q controller was able to decrease the squared plant output response to white noise disturbance by an order of magnitude, outperforming an optimally-tuned positive position feedback controller.Copyright

Interior noise control through tuned loudspeaker absorption

In this work we describe the development and application of an interior noise control approach based on active loudspeaker tuning for sound absorption. Loudspeakers are tuned to minimize global sound pressure at specific acoustic modes through diaphragm velocity feedback. An experimental model of the acoustic space used for controller design is obtained through system identification. H2 optimal controllers are designed to minimize a performance microphone output by feeding back the tuned loudspeaker diaphragm velocity. By sensing speaker velocity rather than far‐field sound pressure, the loudspeaker is actively tuned to minimize reflected sound at specific acoustic modes and therefore reduce the global pressure field. Experimental results, obtained from an acoustic enclosure modeled after a rocket fairing, are used to demonstrate the effectiveness of the tuned loudspeaker at global attenuation. Details of the design procedure and experimental applications are discussed.